U.S. patent number 4,390,595 [Application Number 06/088,269] was granted by the patent office on 1983-06-28 for environmentally protected ir windows.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Frederick G. Yamagishi.
United States Patent |
4,390,595 |
Yamagishi |
* June 28, 1983 |
Environmentally protected IR windows
Abstract
Transparent IR materials having a plasma polymerized saturated
hydrocarbon coating exhibit excellent transmissive characteristics
in the IR and exhibit excellent moisture, oxidative corrosion, and
abrasion resistance.
Inventors: |
Yamagishi; Frederick G.
(Newbury Park, CA) |
Assignee: |
Hughes Aircraft Company (El
Segundo, CA)
|
[*] Notice: |
The portion of the term of this patent
subsequent to May 26, 1998 has been disclaimed. |
Family
ID: |
22210388 |
Appl.
No.: |
06/088,269 |
Filed: |
October 25, 1979 |
Current U.S.
Class: |
428/446; 204/170;
427/160; 428/448; 428/689; 359/359; 427/488; 428/457 |
Current CPC
Class: |
G02B
1/105 (20130101); G02B 1/14 (20150115); Y10T
428/31678 (20150401) |
Current International
Class: |
G02B
1/10 (20060101); B05D 003/06 (); G02B 001/10 () |
Field of
Search: |
;427/38,41,160 ;204/170
;428/446,448,457,689 ;350/1.6,1.7 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hoffman; James R.
Attorney, Agent or Firm: Bethurum; W. J.
Claims
I claim:
1. An environmentally protected optical element for use in infrared
detection systems comprising a transparent substrate having
polished optical surfaces coated with a plasma polymerized
saturated short chain hydrocarbon film which increases transmission
in the 8 to 12 .mu.m region while providing protection against
corrosion and abrasion of said optical surfaces.
2. An optical element of claim 1 wherein said substrate is selected
from the group consisting of germanium, zinc sulfide, zinc
selenide, and silicon.
3. An optical element of claim 1 wherein said hydrocarbon is
selected from the group whose formula is C.sub.n H.sub.2n+2 where n
ranges from 1 to 5.
4. An element of claim 3 wherein said substrate is germanium.
5. An element of claim 4 wherein said hydrocarbon is ethane.
6. An element of claim 3 wherein said substrate is zinc
sulfide.
7. An environmentally protected infrared window comprising a
substrate selected from the group consisting of germanium, zinc
sulfide, silicon, and zinc selenide having surfaces coated with a
thin plasma polymerized saturated short chain hydrocarbon film.
8. An infrared window of claim 7 wherein said film is selected from
the group consisting of methane, ethane, propane, butane and
pentane as the monomer.
9. An infrared window of claim 8 wherein said substrate is
germanium.
10. An infrared window of claim 8 wherein said substrate is zinc
sulfide.
11. An infrared window of claim 8 wherein said substrate is
germanium and said film is ethane.
12. An infrared window of claim 8 wherein said substrate is zinc
sulfide and said film is ethane.
13. A method of providing a protective barrier for the surfaces of
optical elements utilized in infrared detection systems
comprising:
(a) placing said element in an RF discharge reactor adapted for
controllably receiving a gaseous reactant;
(b) operating said reactor at 50-200 watts as gaseous alkane
monomers are passed through said reactor thereby causing said
monomers to polymerize whereby thin films are formed which deposit
onto the surfaces of said elements to provide a transparent
protective coating which prevents corrosion and abrasion; and
(c) subsequently passing a gaseous alkene through said reactor
whereby free radicals, formed from the reaction of said monomers
with said plasma, are quenched prior to exposure of said film
coated elements to ambient environments.
14. The method of claim 13 wherein said element is an IR window
comprised of germanium and said alkane monomer is selected from the
group consisting of methane, ethane, propane, butane and
pentane.
15. The method of claim 14 wherein said element is an IR window
comprised of zinc sulfide and said alkane monomer is selected from
the group consisting of methane, ethane, propane, butane and
pentane.
16. The method of claim 15 wherein said monomer is ethane.
17. The method of claim 13 wherein said monomer is ethane.
18. A process for increasing the transmissivity of radiation
through a selected transparent substrate having polished optical
surfaces while simultaneously protecting said substrate from
erosion and abrasion, which comprises depositing a plasma
polymerized alkane film of a predetermined thickness on one or more
of said surfaces.
19. The process defined in claim 18 wherein said transparent
substrate is selected from the group consisting of germanium, zinc
sulfide, zinc selenide, silver and silicon and wherein said film is
selected from the group consisting of methane, ethane, propane,
butane and pentane.
20. The process defined in claim 19 wherein said substrate is
germanium and said film is ethane.
21. The process defined in claim 19 wherein said substrate in zinc
sulfide and said film is ethane.
22. A process for coating optical surfaces of a selected
transparent substrate to increase the transmissivity of radiation
passing therethrough while simultaneously protecting said substrate
from the degradation caused by erosion and corrosion
comprising:
(a) exposing said substrate to the polymerization of a gaseous
alkane monomer to form a polymerized alkane;
(b) depositing said polymerized alkane on one or more of said
optical surfaces of said substrate and to a predetermined
thickness; and
(c) quenching free radicals which may remain on said substrate
prior to exposure of said substrate to ambient environments.
23. The process defined in claim 22 wherein said substrate is
selected from the group consisting of germanium, zinc sulfide, zinc
selenide, silver, and silicon and wherein said alkane is selected
from the group consisting of methane, ethane, propane, butane and
pentane.
Description
TECHNICAL FIELD
This invention relates generally to the preparation of protective
coatings for optical components and more particularly to the
provision of a plasma polymerized alkane coating for germanium and
zinc sulfide windows used in infrared detection systems.
CROSS-REFERENCE TO RELATED APPLICATIONS
U.S. application Ser. No. 071,605 filed Aug. 31, 1979, by applicant
herein, now issued as U.S. Pat. No. 4,269,896, discloses and claims
surface passivated alkali halide infrared windows and the process
for fabricating the same. The process and materials used to provide
coatings for the alkali halide crystals of application Ser. No.
071,605 are similar to that utilized to coat the germanium and zinc
sulfide windows of the instant application. However, the resulting
effects produced by the present process are significantly
different.
Hughes Aircraft Company is the common assignee of the related
application and the instant application.
BACKGROUND ART
Certain infrared (IR) detection systems are comprised of numerous
optical components designed to reflect and transmit light in an 8
to 12 micrometer wavelength (.mu.m) range. When these systems are
mounted in various vehicles for military and space applications,
they must be provided with optical transmission through their
enclosures and therefore require protective housings which also
transmit light in the 8 to 12 .mu.m range. These systems and their
housings are frequently required to operate at temperatures which
range from a -65.degree. to 165.degree. F. in rain, sleet, snow,
wind, dust and sand. Protection of these systems becomes even more
important if they are used in high-speed aircraft where the
detrimental effects of these environmental conditions are
magnified. These conditions produce abrasion and corrosion and will
degrade the performance of the optical window by erosion and by a
chemical oxidation process. This erosion and oxidation reduces
ultimately the usable lifetime of the window.
In the past, forward looking IR (FLIR) detection systems have been
mounted in enclosures having IR windows fabricated from germanium,
zinc sulfide, and zinc selenide. Germanium is frequently preferred
because of its transmissive characteristics, physical
characteristics and relatively low processing cost. However, the
world supply of germanium is limited and therefore windows of this
material must possess an extended lifetime in severe
environments.
Germanium IR windows and other optical components such as the
silver mirrors are attacked by the environmental conditions in
which FLIR systems operate. These environmental conditions cause
the transmissive character of the optical elements to deteriorate
and necessitates a periodic replacement schedule which, depending
upon the use, may be as often as every month. As a consequence of
this replacement and maintenance necessity, the reliability of the
system is decreased and the cost of these systems is measurably
increased.
Zinc sulfide is the material of choice for windows used as outer
elements in IR seeker systems employed on high-speed aircraft.
These windows deteriorate by erosion caused by dust, sand, and rain
which impinge upon, and ultimately abrade, the surface of such
windows. This phenomenon results in decreased lifetimes of the
windows and increased maintenance costs.
Various attempts to protect IR windows from the deleterious effects
of the environments in which they are exposed have been
unsuccessful. Coatings applied to IR window materials either tend
to preclude IR transmission or fail to protect the crystals which
form such material. The most relevant prior art known to me would
appear to be an article entitled "IR Laser Window Coatings by
Plasma Polymerized Hydrocarbons" by J. M. Tibbitt et al., that was
published in the Proceedings of the Fifth Conference on IR Laser
Windows Materials, Las Vegas, Nev., Dec. 1-4, 1975. Plasma
polymerized ethane (PPE) coatings applied to sodium chloride
crystals were reported in this article to reduce the sensitivity of
these crystals to moisture. However, sodium chloride crystals
having PPE coatings prepared by me in accordance with the teachings
of the above Tibbitt et al. article began to degrade because of
moisture uptake within a relatively short period of time and
exhibited altered IR transmissive spectra.
There are no coatings for germanium and zinc sulfide crystals known
to me which are transmissive in the 8 to 12 .mu.m region of the
light spectrum and which adequately protect against water vapor
and/or condensates, corrosion and abrasion when exposed to these
conditions and phenomenon for relatively long periods of time.
SUMMARY OF THE INVENTION
The general purpose of this invention is to provide environmentally
protected infrared components for IR detection systems.
To accomplish this purpose, I have invented an environmentally
protected optical element for use in infrared detection systems
comprising a transparent substrate having polished optical surfaces
that are coated with a plasma polymerized saturated short chain
hydrocarbon which increases transmission of light in the 8 to 12
.mu.m region while providing protection against corrosion and
abrasion of the optical element.
Germanium and zinc sulfide windows coated in accordance with my
invention exhibit improved transmission in the 8 to 12 .mu.m range
and resist corrosion in a 97-100% relative humidity (RH),
environment at 120.degree. F. indefinitely.
It is therefore an objective of this invention to provide an
improved environmental coating for optical components of infrared
detection systems.
A further objective of this invention is to provide protective
coatings for polycrystalline germanium useful as a material for
optical components in infrared detection systems.
A still further objective of this invention is to provide
protective coatings for zinc sulfide used as optical components in
infrared detection systems.
A still further objective of this invention is to provide
protective coatings for germanium which protect it from
corrosion.
A still further objective of this invention is to provide a
protective coating for a germanium optical element which protects
the optical element from damage due to impingement of solid
particles found in the environment in which said element is
utilized.
A still further objective of this invention is to provide a
protective coating for a zinc sulfide optical element which
protects the optical element from damage due to impingement of
solid particles found in the environment in which said element is
utilized.
These and other objectives and features of this invention will
better be understood and appreciated from the following detailed
description of this invention and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a light transmissive spectrum of PPE coated and uncoated
germanium.
FIG. 2 is a copy of a color photograph taken of a germanium crystal
having a PPE coated surface and an uncoated surface.
FIG. 3 is a copy of photographs taken of a zinc sulfide window
having a partial coating of PPE after sandblast testing.
FIG. 4 is a light transmission spectrum of PPE coated and uncoated
zinc sulfide.
DETAILED DESCRIPTION OF THE INVENTION
I have discovered that plasma polymerized saturated short chain
hydrocarbon coatings provide exceptional protection for optical
elements, utilized in the fabrication of FLIR detection systems,
while enhancing their transmissive characteristics in the 8 to 12
.mu.m range.
While it is generally known that prior art anti-reflective coatings
do not adequately protect the substrates to which they are applied,
I have discovered that PPE films or coatings increase the
transmission of germanium and zinc sulfide in the 8 to 12 .mu.m
region and, at the same time, protect against corrosion and
abrasion.
An example of the anti-reflection character of a 2 .mu.m PPE film
applied to germanium is shown in the light transmission spectrum
presented in FIG. 1. In this instance, a percent transmission
increase of as much as 27% over uncoated germanium in the 8 to 12
.mu.m region was obtained. However, should increased corrosion or
abrasion protection be desired, the film thickness may be increased
by odd multiples of the quarter wave to obtain an optimum product.
Similarly, PPE will act as an anti-reflective coating on zinc
sulfide if the appropriate film thickness is used.
Infrared windows fabricated from transparent germanium metal
crystals coated in accordance with my invention have shown no signs
of degradation after exposure to a 97-100% RH (120.degree. F.)
environment for more than 11 months (testing was arbitrarily
terminated). The transmissive characteristics of these windows are
superior to those of uncoated germanium which require replacement
within one month under similar conditions.
In fabricating windows for use in the 8 to 12 .mu.m range, I prefer
to utilize ethane as the monomeric source for my coatings. However,
other saturated short chain alkane monomers such as methane,
propane, butane, and pentane are also useful.
Unlike other prior art coatings, plasma polymerized ethane (PPE)
forms a protective barrier about the uncoated substrate which
prevents corrosion and at the same time exhibits anti-reflection
properties.
Polycrystalline germanium, unlike the alkali halide crystals of my
copending application Ser. No. 071,605 which dissolve in a moisture
rich environment, undergoes a detrimental chemical reaction when
exposed to oxygen in a moisture rich environment. The coatings of
this invention are believed to prevent the oxidative reaction from
occurring by eliminating moisture as a catalyst. The extent of the
protection afforded by the thin films of this invention is
astounding in view of their thicknesses which range from 2 to 8
.mu.m.
Referring to FIG. 2, oxidation and pitting of the uncoated surface
of germanium stored in a 97-100% RH environment at 120.degree. F.
for eleven months is evident from the light blotches and spots seen
in the plate on the right. However, the coated surface of the same
material showed essentially no change over the same storage period.
There is evidence (several small spots) of the formation of small
bubbles beneath the surface of the film on the coated side (left
plate of FIG. 2) of the germanium. However, the overall quality of
the window remained good.
Plasma polymerized short chain hydrocarbon films and coatings have
been applied to other materials used as optical elements in IR
detection systems with excellent results. For example ZnS samples
coated with PPE are substantially more abrasion resistant than
uncoated ZnS. FIG. 3 illustrates the abrasion resistance of PPE to
fine-grain sandblasting. A zinc sulfide crystal having PPE coated
and uncoated surfaces was exposed to fine-grain sandblasting for
one minute and subsequently photographed with an optical microscope
at 26X. The plate on the left shows severe pitting of the uncoated
surface in the upper half and left quarter portion of the plate.
The lower right quarter portion of the plate is a shot of the
uncoated crystal surface that was protected by the metal tool used
to hold the sample. The top half of the plate in the center
similarly shows pitting of the uncoated crystal surface while the
lower half of the center plate shows the protection afforded by the
PPE coating to the surface of the crystal.
It is clear, upon examination of the center plate, and upon
comparing the protected portion of the crystal with the unprotected
portions, that the PPE film provides excellent protection of the
crystal surface. The plate at the right show a portion of the
protected crystal surface after the sandblasted PPE film was
removed. The black striations in the surface of the crystal in the
plate on the right were caused by scraping of the surface with a
metal blade to remove strongly adherent PPE film in order to
examine the surface protected by the film.
Similarly one would expect silver mirrors coated with a thin film
of plasma polymerized hydrocarbon to resist corrosion for
indefinite time periods. Plasma polymerized films may be applied to
ZnSe and Si substrates as well.
The infrared transmitting protective films presented in this
invention were prepared by a process called plasma polymerization.
This term generally describes the use of several types of electric
discharge configurations in which molecules (from gaseous monomers)
are subjected to energetic electrons in the discharge or plasma.
This results in the formation of intermediate free radicals, ions,
and other high energy species derived from the monomer. Interaction
of these species to form plasma polymerized films is not completely
understood. However, it is generally believed that the initial step
is the absorption of the monomer on the substrate surface. This
monomer layer is then bombarded by reactive plasma species as well
as being acted upon by photochemical energy produced in the plasma.
There is little discrimination shown in the position of formation
for the free radicals. The net result of the propagating step is
both a continuing growth of the polymer chains and a developing
matrix of crosslink sites.
Following termination of the plasma there are long-lived residual
free radicals trapped in the bulk and on the surface of the film.
These radicals can react with atmospheric oxygen and must be
quenched before the film is exposed to air so that polar oxygen
containing groups, which would be absorbing in the infrared, are
not formed. An ethylene gas purge is effective for this purpose.
Other unsaturated alkene gases would also be effective for this
purpose.
Polymers prepared via this method can vary in structure and
molecular weight depending upon the reaction conditions. The
polymer can be prepared as an oil, powder or film. In order to
obtain uniform films the reaction parameters of monomer flow rate,
reaction pressure, discharge power, substrate surface preparation,
and reactor configuration need to be optimized for each monomer.
Thus, uniform, highly crosslinked films may be conveniently
prepared. For a given set of reaction conditions the deposition
rate is constant and any desired thickness can be obtained by
operating for a predetermined time.
The PPE film used in this invention was prepared in a capacitively
coupled RF discharge at 13.56 MHz in a Pyrex tubular flow reactor
using 2 inch by 3 inch parallel plate copper electrodes separated
by 11/4 inch. Other reactor configuration can be use also. The
sample, in this case polished polycrystalline germanium or zinc
sulfide, was placed on the bottom electrode and the system was
evacuated to 0.05-0.01 Torr. Ethane was bled into the system and
the pumping speed was adjusted by a valve so that the pressure in
the system was steady at the desired value. The plasma was
initiated and the power output of the RF generator was
simultaneously matched to the plasma load via an impedance matching
network. After a film of desired thickness was obtained the
discharge was terminated, the flow of monomer was discontinued and
the reactor evacuated. Residual free radicals were quenched with
ethylene prior to exposing the window to the air. Deposition rates
were determined by depositing the film on glass cover slides and
measuring the thickness on a Dektak FLM, Sloan Technology Corp.,
Santa Barbara, Calif.
Specific examples of this invention and its testing are provided
below:
EXAMPLE I
Plasma polymerized ethane was deposited on a polished piece (1"
diam..times.2 mm thick) of polycrystalline germanium under the
following conditions: Initial pressure=0.02 Torr, flow rate of
ethane (Matheson Gas, CP grade)=10 ml/min @ STP, reaction
pressure=2 Torr, power=50 watts (continuous mode) (Tegal Corp., RF
Generator, Model 300P and impedance matching system, Richmond,
Calif.). After 8 hours the reaction was stopped giving a slightly
yellow film .about.6 .mu.m thick. The sample was stored under 15
Torr of ethylene for 15 hours before exposing it to the air. The
infrared spectrum showed a 5% transmission loss at 10.0 .mu.m since
this thickness is close to a half-wave. A 2 .mu.m thick film which
is close to a quarter-wave prepared under similar conditions shows
a 12% increase in transmission over an uncoated crystal as shown in
FIG. 1. The index of refraction of PPE in this case was found to be
1.35.+-.0.1 by measuring the optical thickness in the infrared.
EXAMPLE 2
The germanium window coated with PPE (example 1) was subjected to a
standard salt spray test for 24 hours without any visible damage.
However, an uncoated sample was also undamaged. The coated window
was then placed in a 97-100% relative humidity chamber at
120.degree. F. The uncoated germanium window began to fog and
became pitted after 30 days. After 37 days the damage was severe.
The coated side side, however, was not damaged after eleven months
of storage.
EXAMPLE 3
Plasma polymerized ethane was deposited on a polished piece of
polycrystalline zinc sulfide (Kodak, IRTRAN-2, 1 cm.sup.2) under
the same reaction conditions used in Example 1. After 3.5 hours the
reaction was stopped and the sample was stored under 15 Torr of
ethylene for 15 hours. This process gave a slightly yellow film
.about.3.8 .mu.m thick. The IR spectrum of this coated optical
component is shown in FIG. 4. The anti-reflective property of the
coating can be maximized for a desired wavelength region by using
the appropriate film thickness.
EXAMPLE 4
The zinc sulfide window coated with PPE (Example 3) was subjected
to 98% RH at 20.degree. for 25 hours with no change in the IR
spectrum (see FIG. 4). It was then exposed to 97-100% RH at
120.degree. F. for 8 months at which time the testing was
arbitrarily stopped. The center portion of the coated window was
undamaged although there was some indication of peeling around the
edges.
EXAMPLE 5
A 7.7 .mu.m thick film of PPE was deposited on a portion of a 1
cm.sup.2 piece of polycrystalline zinc sulfide by the process shown
in Example 1. A portion of the uncoated substrate was masked with a
pair of forceps and the entire piece was exposed for 1 minute to
fine-grain sandblasting at 65 PSI pressure with the nozzle
perpendicular to the surface at a distance of 8 inches. The PPE
film showed excellent resistance to abrasion caused by sandblasting
as shown in FIG. 3.
INDUSTRIAL APPLICABILITY
Plasma polymerized coatings of saturated hydrocarbons applied to
transparent IR materials in accordance with this invention yield
environmentally protected optical components which are suitable for
use in infrared detection systems that are exposed to military and
space environments. The used of these materials will appreciably
reduced the cost of infrared detection systems by eliminating the
necessity for replacing optical elements at a high frequency. Laser
windows and other optical elements may be treated in accordance
with this invention to provide protection from moisture, corrosion
and physical abrasion.
Having disclosed my invention and provided teachings which enable
others to make and utilize my invention, the scope of my claims may
now be understood as follows.
* * * * *